As technology evolves and mobile devices such as smartphones and tablets become increasingly commonplace in a wide variety of environments including retail as well as medical settings, among others, there is a growing demand for mobile devices that can capture high quality images using only the camera available on such devices. In doing so, computing device, camera, imaging system, consultation screen and communications device, can all be merged into one portable device.
The ability to capture high quality images is particularly important for applications in which the images are to be further processed and analyzed, such as by image processing systems for identifying, characterizing, or measuring various conditions present in the images. One such application, for example, is in providing skin care consultation or treatment. This often entails the identification and characterization of certain skin conditions, some of which may not even be visible with the naked eye. If unable to minimize the effects of ambient lighting on the subject, the various algorithms and image processing techniques involved in such applications will provide substandard or erroneous results. Additionally for many such applications, standardization from image to image is important, particularly where images are to be compared, such as sequential images of subjects before and after treatment.
To achieve acceptable results, high-end DSLR cameras and well-controlled illumination environments have been typically used. For example, the VISIA system from Canfield Scientific employs a DSLR camera and multiple flash illumination sources within a booth-like enclosure to capture and analyze high quality facial images.
While the cameras built into tablets or other mobile devices are by no means equivalent in quality to high-end DSLR cameras, they have certainly made significant advances since their initial integration into such devices. One of their biggest drawbacks, however, is the lack of a high quality flash illumination source.
Flash photography has been around for over a century and relies on the principle that by creating an intense burst of light in a fraction of a second, the effects of ambient lighting can be negated or at least temporarily reduced as a photographic image is captured. Xenon flashes have been commonly used as illumination sources in flash photography.
While some mobile devices have LED “flashes” integrated therein, these are not nearly as bright as a typical xenon flash and do not operate in a similar manner. Rather than being pulsed for a fraction of a second to emit an intense burst of light, the LED flash of a mobile device is typically turned on as an image capture sequence begins and remains on for the entire process. The LED “flash” of a typical mobile device simply cannot match the performance of a xenon flash to minimize the effects of ambient lighting.
An additional complication arises from the use of a rolling shutter, typical of cameras used in mobile devices. With a rolling shutter, a still picture (in a still camera) or each frame of a video (in a video camera) is captured, not by taking a snapshot of the entire scene at a single instant in time, but rather by rapidly scanning across the scene, either vertically or horizontally. In contrast to a “global shutter,” in which the entire frame is captured at the same instant, with a rolling shutter not all parts of the image of the scene are recorded at exactly the same instant. As such, the scene can change as it is recorded piecemeal. During playback of a captured image, however, the entire image of the scene is displayed at once, as if representing a single instant in time. As a result, images captured by rolling shutter cameras will display predictable distortions of fast-moving objects or rapid flashes of light.
Additionally, if illuminated using a flash that is not properly synchronized with the rolling shutter, a captured still image may appear to have two distinctly delineated portions of markedly different brightness. This problem does not occur with the LED “flash” of a typical mobile device camera because the LED is typically turned on as an image capture sequence begins and remains on for the entire process. As mentioned, however, such LED light sources simply cannot match the brightness of a xenon flash which is activated for a much shorter period of time.
One possible approach to address this problem would be to use an external xenon flash. For proper operation, however, precise synchronization of the external flash with the operation of the camera integrated into the mobile device is essential. Access to the circuitry that controls the image capture functions of a typical mobile device, however, is severely limited or non-existent, thereby making it difficult to interface an external flash to a mobile device.
The lack of a high quality flash illumination source is clearly a limiting factor in any attempt to use a mobile device camera to provide the level of image quality and standardization that is required for many applications, such as those mentioned above.